Alien is the Frankenstein for the modern movie-goer. Concerned with themes of science and progress, and the prospect of unknown threats, the movie kicks against the naturalist, scientific philosophies that keep us fearless in the technological world. The film reminds its viewer that the world is still scary and that there is an unknown thing that goes bump in the night.
The film, regarded as a powerful part of pop-culture is not just pulp. There are many artistic aspects of the film involving story-telling and camera work. Consider the scene where Dallas is in the air vents. The camera work itself is framed tightly, making the viewer feel claustrophobic and scared of what might be in the darkness just behind the captain. Then you see the thing by the light of the torch, perched and aggressive, ready to attack. This artistic aspect of the film is convincing to the audience, emphasizing the films themes of the unknown in a way that resonates with the audiences personal experience watching the film.
There is also a lot of packed symbolism. For instance, the traitor on board is literally a corporate robot. Another symbolic aspect, perhaps more subtle, is that the monster is described by Ash as "Kane's son." This is a dual homage--first to Frankenstein which also portrays its monster as a bastard son. Both Frankenstein's monster and the Alien are instances of misplaced curiosity. Both are brought about by the scientific endeavor. The second allusion is to the Bible, where the son of Adam who kills his brother is named Cane. In literature references to Cane' son are often employed to emphasize the evil inherent in human nature. Cf. Steinbeck's East of Eden.
This is further emphasized by the fact that the alien, though foreign, is born from within a human body. This dual origin is also meaningful, perhaps also alluding to a Judeo-Christian understanding of evil and human morality, namely in that the evil thing is not part of our design, but nevertheless is born from within us.
Alien stands as a beautiful horror film because all of its facets reinforce its core argument in a way that evokes fear and appreciation in the viewer, emphasizing the fear in 'pity and fear'. It is a movie about our fear of the dark, our dread of the unknown, and it makes us hyper-aware of all the unknowns we daily ignore: the unpredictability of human nature, the failure of science and technology, the fear of the unknown and our ultimate fate.